Selecting right paint for steel provides protection against corrosion assures that the most economical and best technical solution is achieved.
1. Introduction
A weak painting system might be cheaper at first but additional maintenance cost in the lifetime of the steel buildings might boost the overall cost. On the other hand an excess of paint for a very low corrosion environment will be a mismanagement of the construction cost planning.
2. Corrosion Category
Identifying the corrosion category of the environment is the most important factor to consider before selecting a right paint for steel buildings.
When selecting a paint system it is vitally important to define the conditions in which the structure, facility or installation is to operate. To establish the effect of environmental corrosion, the following factors must be taken into account:
• humidity and temperature (service temperature and temperature gradients)
• the presence of UV radiation
• chemical exposure (e.g. specific exposure in industrial plants)
• mechanical damage (impact, abrasion etc)
The nature of the environment and the corrosive contributing conditions will have an effect on:
• the type of paint used for protection
• the total thickness of a paint system
• the steel surface preparation required
• minimum and maximum recoating intervals
ISO 12944 distinguishes 6 basic atmospheric corrosion categories:
| C1 | Very Low |
|---|---|
| C2 | Low |
| C3 | Medium |
| C4 | High |
| C5 | Very High |
| C6 | Extreme |
The characteristics of the above categories are as follows;
Corrosion Category |
Exterior |
Interior |
|---|---|---|
| C1 | – | Heated buildings with a clean atmosphere such as offices, shops, schools, hotels. |
| C2 | Atmospheres contaminated to a small extent, mainly rural regions | Buildings which are not heated, where condensation may occur e.g. storage facilities, sports halls. |
| C3 | Industrial and urban atmospheres with a low sulphur oxide (IV) contamination level. Inshore areas of low salinity. | Production halls to facilities humidity and certain air contamination e.g. foodstuff plants, laundries, breweries, airies. |
| C4 | Industrial areas and inshore areas of medium salinity | Chemical plants, swimming pools, ship repair yards. |
| C5 | Industrial areas of high humidity and aggressive atmosphere and inshore areas of high salinity. | Buildings and areas of almost constant condensation and high contamination. |
For steel constructions, we set the relevant corrosion category together with our clients and determine the paint system according this category.
3. Novelty Steel Paint Solutions
- Alkyd Coatings
Alkyd paints are the modern descendant of oil paints. Instead of pigment suspended in oil, alkyd paints are typically formed by an alkyd resin dissolved in a thinner. These one-component primers are easy to apply and economical but offer limited rust prevention in the long run. For low corrosive environments, these paints can be used for budget purposes.
Figure 1: Steel Painting
- Epoxy Coatings
Epoxy coatings are typically comprised of an epoxy base and a curing agent. A wide variety of coating properties can be achieved by manipulating either of these components: Epoxy polyamide coatings offer great moisture resistance, epoxy mastic coatings offer exceptional film thickness and phenolic epoxy coatings offer good chemical resistance. And due to this versatility, you’ll find epoxies used as a primer, intermediate coat or even a topcoat depending on the needs of the application.
Figure 2: Steel Painting-2
Advantages
- Abrasion resistant
- Chemical resistant
- Good performance when submerged
- Can easily build film thickness for enhanced abrasion resistance
- Can be formulated to exhibit a wide variety of favorable coating properties
Disadvantages
- Chalks when exposed to UV light
- Low flexibility
- Polyurethane Coatings
Polyurethane coatings are widely used —often as a topcoat— in applications where durability and abrasion resistance are key considerations. Most often, polyurethanes are chosen as the topcoat of a total protective coating system. For example, polyurethane might be applied as a topcoat above a zinc-rich primer and epoxy intermediate coat. Polyurethanes are versatile enough to be formulated to handle a wide variety of service environments.
Figure 3 : Steel Painting-3
Advantages
- Abrasion resistant
- High gloss and color retention
- Aliphatic polyurethanes perform well against weathering and UV light
- Aromatics perform well when submerged
- Low VOC formulations available
Disadvantages
- Contains isocyanate (-NCO) ,a harmful carcinogen
- Skilled workers and protective equipment are necessary for application
- Higher priced than epoxies
- Zinc rich Coatings
As a generic coating type, zinc-rich coatings refer to organic or inorganic coatings with high loadings of zinc dust. The zinc provides galvanic protection of the steel surface, meaning that it will corrode instead of the steel beneath it. As the zinc-rich coating corrodes it forms a barrier between the steel and its environment
Zinc-rich primers will often be applied as part of a two-coat (zinc-rich primer, polysiloxane topcoat) or three-coat (zinc-rich primer, epoxy intermediate coat, polyurethane topcoat) system.
Advantages
- Provides both galvanic and barrier protection to steel
- Abrasion resistant
- High level of durability
Disadvantages
- Must be top coated in many applications
- Both inorganic and organic variations have low resistance to acid and alkali
4. Conclusion
The prevailing service environment —along with factors such as cost, accessibility of the asset, expected labour costs and desired service life of the coating system— will define which coating system is best-suited for your application.
Coating selection happens on a case by case basis, and we can evaluate the specific needs of your application, offering several systems that could work based on our decades of experience in the industry.
Contact our team to get more information about the coating options of steel structures.
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